This disclosure relates to methods of making bisphenol employing an acidic ion exchange resin. In particular, this disclosure relates to methods of making bisphenol employing water addition to the reactants.
Typical bisphenols, such as 4,4′-isopropylidenediphenol, e.g., bisphenol-A (BPA), are widely employed as monomers in the manufacture of polymeric materials, such as engineering thermoplastics. For example, BPA is a principal monomer used in the manufacture of polycarbonate. Bisphenols are generally prepared by the electrophilic addition of aldehydes or ketones such as acetone to aromatic hydroxy compounds such as phenol, in the presence of an acidic catalyst composition. These types of reactions are also referred to as acid catalyzed condensation reactions. While commercially sulfonated polystyrene resin cross-linked with divinylbenzene, (PS-DVB) is typically used as the solid acid component of the catalyst composition, other acidic catalysts are also known to be effective in bisphenol synthesis. Zeolites, metal oxides, polysiloxanes, and other acid catalysts on organic and inorganic supports have been described as effective bisphenol catalysts. Reaction promoters can also be employed as part of the catalyst composition to improve the reaction rate, and selectivity, of the desired condensation reaction; in the case of BPA, the desired selectivity is for the para—para isomer. Promoters can be present as unattached molecules in the bulk reaction matrix, e.g., “bulk-promoters”, or can be attached to the resin through ionic or covalent linkages, e.g., “attached-promoters”. A useful class of promoter is the mercaptans, specifically thiols, e.g., organosulfur compounds which are derivatives of hydrogen sulfide.
Most catalysts that are used to produce bisphenols not only catalyze the desired condensation reaction to form bisphenol, but also catalyze isomerization reactions between bisphenol and other byproducts. While it is desirable to contact the reactants with the catalyst for sufficient time to maximize the production of bisphenol, one must also be careful not to have contact times be so large that the conversion of bisphenol to undesired products predominates. In other words, there is an optimum contact time, leading to an optimum concentration of bisphenol. In a continuous reactor, the contact time is the residence time in the reactor. Alternatively, the inverse of the residence time, known as the weight hourly space velocity (WHSV) can be used, where WHSV is defined as the weight of flow through the reactor bed per the weight of the catalyst per hour. It is normally desired to maximize the conversion to bisphenol (i.e., maximize selectivity) in order to minimize the amount of raw materials required to produce a unit of bisphenol.
Another complicating factor can be that the catalyst activity declines with time. This is a well known effect with acidic catalysts used in the manufacture of bisphenol. Since the catalyst activity declines, the effective contact time decreases (or the effective WHSV increases during the lifetime of the catalyst). Typically, in a commercial reactor, sufficient catalyst is loaded into the reactor to allow for a given production rate at the end of the catalyst life. However, the initial activity of the catalyst is high, and can lead to high conversions of reactants to bisphenol, and because of the series nature of the reaction, can also lead to isomerization of bisphenol to undesired by-products. In other words, the initial selectivity of the reaction can be lower than desired.
In addition to the problem discussed above, there are often circumstances in a chemical plant where it is necessary to adjust the reactors to produce less than the maximum amount of bisphenol. Since the amount of catalyst in the reactor is fixed, and reducing production requires the flow rate through the reactor to be decreased, the net effect is to decrease the WHSV, and to increase the residence time or the contact time. According to the discussion above, this increase in contact time can lead to more of the bisphenol being converted to by-products.
Accordingly, there is a need for a method to maintain selectivity over a range of flow rates and catalyst activities.